1. Field of the Invention
The present invention relates to the electrodeposition of iridium/iridium oxide onto the surface of a microelectrode. More particularly, the invention relates to an improvement in the electroplating of iridium onto the surface of a microelectrode comprising a transition metal or mixtures thereof. The electroplated microelectrode is capable of holding and transmitting a higher charge density in biomedical applications than presently available plated microelectrodes. These microelectrodes are particularly useful when used in conjunction with electrical devices to treat neurological diseases and conditions in living mammals.
2. General Description of the Field
The use of electrical stimulation of muscles and nerves in the body to overcome specific diseases and nerve conditions has been under experimentation for a number of years. The diseases and conditions include--hearing loss (cochlear implant), incontinence, or impotence (series of implanted electrodes), heart arrhythemia (pacemaker) retinal stimulation, spasticity, limb paralysis, and the like.
Although much of the early development was empirical, it was recognized that the implanted electrodes need specific desirable characteristics. First, the basic electrode material needed to be non-toxic. That is, with or without the electrical activity, the implanted metal did not cause tissue or nerve damage or necrosis in the short or long term. Second, the precise form of the electrical stimulations needed to insure that any electrical charge injected into living tissue be balanced to prevent any irreversible reactions which would dissolve or impair the electrode. It was found that copper, stainless steel, silver or other generally common electrode materials rapidly corrode when electrically charged in an electrolyte environment, such as body fluids. In the early research, certain metals were identified as generally being an acceptably low corrosion rate so long as the charge density was limited to 200 microcoulombs/cm.sup.2 or less. Generally, these electrode materials include, for example, platinum, gold, iridium, rhodium, palladium, mixtures (or alloys) of these and the like.
For the stimulation of large-scale muscles and nerves, electrodes of the above metals or alloys were reasonably large in size; therefore, it was possible to keep stimulation parameters well within the charge density requirements. However, with the development of neural prosthetic devices for delicate structures such as the inner ear, eye, etc., microelectrodes smaller than those of the art were required. These microelectrodes and the electrical current density which was required to be transmitted through them quickly pushed to the limit the safe charge carrying capacity of the above-described metals and alloys in their present configurations.
In such delicate applications where the charge capacity required for electrical stimulation might be as high as 200 microcoulombs per square centimeter, the present microelectrodes are being driven dangerously close to the limit where irreversible dissolution and gas evolution occurs. The trend of the research was to go to much denser and smaller electrodes.
There are several known methods of increasing the capacity of a metallic electrode to carry and transfer an electrical charge. Since the charge is only safely transferred by the chemical reactions in which all products are insoluble and remain bonded to the electrode surface, the electrode charge capacity can be increased by identified electrode-bound reactions involving more electron transfers, i.e., valence states of the metallic oxides, Alternatively, discovering a method of increasing the real effective surface area of an electrode will allow more charge to safely flow through it.
The above described chemical design considerations are complicated and difficult, generally because material which may be optimal as an electrode for its mechanical properties may be far from optimal in terms of its electrical, chemical and biochemical properties. Specifically, a number of research groups have established that iridium and its oxides have more valence states than other metallic oxides, and it represents a greatly improved electrode interface as compared with platinum. However, iridium itself is generally not mechanically suitable as a material for a microelectrode. Some reports have been made about mechanically coating iridium onto the surface of platinum wires (which have good electrical and mechanical properties) by dipping in iridium chloride solution followed by heating (baking at 325.degree. C. or higher) the iridium coating at elevated temperatures. This technique often resulted in greatly increased charge capacity of the microelectrode, but the iridium coatings were not predictable either electrically or mechanically. Iridium was electroplated onto the platinum electrode using conventional direct current (DC) electroplating techniques. These DC plated electrodes had increased in charge capacity, but the iridium coatings were not mechanically rugged. After being exposed to ultrasonication) (a conventional cleaning and testing technique), the charge capacities of the electrodes were very unpredictable. The fundamental problem underlying the lack of mechanical ruggedness is that the mechanical and chemical adhesion between the base platinum electrode and the iridium metal coating is generally not very good.
It is therefore desirable to have a technique which will produce a microelectrode having improved adhesion between the iridium coating and the base electrode and have predictable mechanical ruggedness to withstand the electrical, chemical and biological environments to which it will be subjected during use. It is also desirable to have methods available to condition iridium-coated microelectrodes to increase the overall charge density.